The mechanical properties of crustacean muscle have received little attention despite their importance in posture and movement. Measurements of the isometric length-tension relationship and its relationship to the working range of the muscles, the isotonic force-velocity relationship, the stiffness of the series elastic element, the fatiguability of the muscle, and the dynamic stiffness under isovelocity conditions will be obtained for homologous postural muscles in crayfish, lobster and hermit crab. Each muscle will be examined to determine the proportions of muscle fibers with different morphological (sarcomere length, length of A and I bands, thin-to-thick filament ratio, density of dyads) and histochemical (oxidative capacity, actomyosin ATPase activity) characteristics. My working hypothesis is that muscle fiber diversity insures that a specific muscle can operate efficiently as a linear elastic element over its physiological range, as well as providing the necessary speed of contraction and resistance to fatique described by other workers.